Ceramic nuclear fuel or breeder material tablets and method of producing the same

ABSTRACT

Ceramic fissionable fuel or breeder material tablet axially alignable with corresponding tablets for forming nuclear reactor fuel rods includes a substantially cylindrical member having a depression at opposite end faces thereof, the end faces respectively comprising a marginal zone of slightly sloping conical shape having an angle of inclination within a range of 0.5* to 30*, the inclined marginal zone defining, with the remaining surface of the respective end face, an annular contact edge for contacting an end face of another tablet axially alignable therewith, the annular contact edge being spaced from the peripheral edge of the end face a given distance so that compression forces acting on the annular contact edge when the tablet is in use in a nuclear reactor fuel rod, are absorbable by a spatial region of the table having adequate strength for absorbing the compression forces.

United States Patent [191 Schart Apr. 2, 1974 [75] Inventor:

[73] Assignee: Siemens Aktiengesellschaft, Berlin,

Germany [22] Filed: Feb. 14, 1972 [21] Appl. No.: 226,298

' Related US. Application Data [63] Continuation of Ser. No. 885,692,Dec. 17, 1969,

Hans Scharf, Nurnberg, Germany abandoned.

[30] Foreign Application Priority Data Dec. 20, 1968 Germany 1815957176/90 [51] Int. Cl G21c 3/02, G21c 3/30 [58] Field of Search 176/66,73, 74, 90, 91

[56] References Cited UNITED STATES PATENTS 3,140,983 7/1964 Waine176/73 3,184,392 5/1965 Blake 176/73 X 3,192,621 7/1965 Bauer et al.176/73 X 3,227,622 1/1966 White 176/73 3,322,644 5/1967 Benson 176/733,365,371 1/1968 Lass et a1... 176/74 X 3,415,911 10/1968 Lloyd 176/73 X3,022,240 2/1962 Bassett 176/74 X FOREIGN PATENTS OR APPLICATIONS958,697 5/1964 Great Britain 176/73 971,930 10/1964 Great Britain l176/74 884,969 12/1961 Great Britain 176/73 893,742 4/1962 Great Britain176/90 Primary Examiner-Carl D Quarforth Assistant Examiner-Roger S.Galthier Attorney, Agent, or Firm-Herbert L. Lerner [57] ABSTRACTCeramic fissionable fuel or breeder material tablet axially alignablewith corresponding tablets for forming nuclear reactor fuel rodsincludes a substantially cylindrical member having a depression atopposite end faces thereof, the end faces respectively comprising amarginal zone of slightly sloping conical shape having an angle ofinclination within a range of 05 to 30, the inclined marginal zonedefining, with the remaining surface of the respective end face, anannular contact edge for contacting an end face of another tabletaxially alignable therewith, the annular contact edge being spaced fromthe peripheral edge of the end face a given distance so that compressionforces acting on the annular contact edge when the tablet is in use in anuclear reactor fuel rod, are absorbable by a spatial region of thetable having adequate strength for absorbing the compression forces.

6 Claims, 5 Drawing Figures LiBUlASl MENTEU APR 2 I974 CERAMIC NUCLEARFUEL OR BREEDER MATERIAL TABLETS AND METHOD OF PRODUCHNG THE SAME Thisis a continuation, of application, Ser. No. 885,692, filed Dec. 17,1969, now abandoned.

My invention relates to ceramic nuclear fissionable fuel or fissionablematerial tablet and more particularly to such tablet having acylindrical form with dished end faces and which are used for theconstruction of nuclear reactor fuel rods.

A complete nuclear reactor fuel rod is formed of a tubular can or jacketconsisting, for example of zirco nium or steel, which surrounds a columnor a series of axially aligned tablets consisting of fissionable fuel orbreeder material. These tablest of predominantly ceramic, primarilyoxide ceramic materials, are produced conventionally by the compressionof suitable powders in a cylindrical mold. These pressed articles,so-called green bodies are then sintered and thereby attain the desiredcharacteristics, for example, with respect to strength, density,porosity, etc. In the course of the sintering process, a volumereduction of about to percent of the original volume occurs.

Due to frictional conditions during the compression step and theinhomogeneities in density connected therewith, this volume reductionoccurring during the sintering step does not take place equally anduniformly. Shaped members are produced rather whose original cylindricalouter surfaces have a similarity to a rotational hypeboloid and whoseinitially flat end faces are curved inwardly so as to be slightlyconcave. In this form, the tablets cannot be used however in manysituations. They must first be given, by grinding operation, the desiredcylindrical shape and nominal dimension. To absorb the very largethermal expansion of the central regions of the tablets during operationof the nuclear reactor within which they are located, the tablets areoften provided with depressions, so-called dishing in the end facesthereof, which is previously effected during the compression step.

The sharp edges which are formed by the manufacturing step at the rimsof the tablets are very pressuresensitive which is especiallydisadvantageous when inserting the tablets into the tubular cans orjackets and even when only slightly bending the fuel rods. The edges ofthe ceramic material which often crack or rupture sporadically duringthe manufacturing steps not only disrupt the construction of thefissionable fuel columns but can also cause damage to the tubular can orjacket which, under certain conditions, is of very thinwalledconstruction. These phenomena can be readily explained in that only apoint contact often occurs be tween the individual tablets so that theultimate or breaking strength of the ceramic is exceeded by relativelysmall forces due to the point contact. To counteract this effect it hasalready been suggested that the fracture-sensitive edges of the tabletsbe blunted through grinding a marginal chamfer thereon. In addition tothe rather expensive production process, this suggested measure does notlead to the desired result because the engagement or contact betweentablets then only occurs in the direct vicinity of the margins of thetablets.

It is accordingly an object of my invention to provide ceramic nuclearfuel or breeder material tablets and method of producing the same whichaffords a considerable improvement in processability of the tabletswithout increasing the cost of manufacture thereof. More particularly,it is an object of my invention to provide such tablets which will besubject to minimal fracture.

With the foregoing and other objects in view, I provide in accordancewith my invention ceramic fissionable fuel or breeder material tabletaxially alignable with corresponding tablets for forming nuclear reactorfuel rods, comprising a substantially cylindrical member having adepression at opposite end faces thereof, the end faces respectivelyincluding a marginal zone of slightly sloping conical shape having anangle of inclination within a range of 0.5 to 30, the inclined marginalzone defining, with the remaining surface of the respective end face, anannular contact edge for contacting an end face of another tabletaxially alignable therewith, the annular contact edge being spaced fromthe peripheral edge of the end face a given distance so that compressionforces acting on the annular contact edge, when the tablet is in use ina nuclear reactor fuel rod, are absorbable by a spatial region of thetablet having adequate strength for absorbing the compression forces.This means that in accordance with the strength of the tablets, which isdependent upon the type of material, the contacting edge is spaced from0.5 to 2 mm from the peripheral surface of the tablet. This is notaccompanied by any difficulties in the manufacture thereof because thisinclination of the end faces can be produced by a suitably shapedcompression die when compressing the tablet, the shape of the thus compressed body being such as to have the desird sloping conical shape withan angle of inclination within the range of 0.5 to 30 after contractionof the compressed body in a subsequent sintering step.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin ceramic nuclear fuel of breeder material tablets and method ofproducing the same, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a heretofore known device forcompressing a known tablet as a first stage in the formation thereof;

FIG. la is a sectional view of a heretofore known tablet produced by theknown device of FIG. 1;

FIG. 2 is a view similar to that of FIG. 1 of a device for producing thetablet of the invention in the instant application;

FIG. 2a is a sectional view of the tablet of the instant invention; and

FIG. 3 is a fragmentary longitudinal sectional view of a fuel rod formedof tablets according to the invention.

Referring now to the drawing, and first particularly to FIG. 1 thereof,there is shown a known device for performing the compression step on aconventional nuclear fissionable fuel tablet to be sintered. The tablet10 is provided with a flat end face on one side thereof, and is providedwith an impressed dishing at the end face on the other side thereof.Accordihg to FIG. 1, the tablet is initially formed as a green body bymeans of compression dies 2 and 3 shaped to produce the ultimate form ofthe tablet. FIG. 1a shows compressed tablet 1 of FIG. 1 after it hasbeen subjected to a sintering step. Thus, the finished fuel tablet 10 ofFIG. 1 produced by the known device and method is formed with acurvature both in the peripheral surface thereof as well as in the endface thereof which was originally formed with a flat surface in thecompression step shown in FIG. 1. By subsequent grinding along thedotted lines 5 and 9 in FIG. la, the tablet produced by the known methodand device must then be brought to the nominal dimension and the nominalshape, a process which is rather costly, especially for the end faces.

In contrast thereto, there is shown in FIG. 2 the method of productionof a nuclear fissionable fuel or breeder material tablet according tothe invention of the instant application. The compression dies 2 and 3in FIG. 2 are shaped in such a manner that the marginal zone of thefissional fuel or breeder material tablet 4 is provided with aninclination. The angle of inclination is indicated by a. After sinteringand the subsequent contraction of the tablet 4 of FIG. 2, it assumes theshape of the tablet 40, shown in FIG. 2a. It is apparent therefrom thatthe angle or decreases in size to the value a. This is produced due tothe contraction of the tablet in axial direction during the sinteringstep. Since the contraction displacement for the particular material isvirtually uniform, the size of the angle a can be adjusted in thecompression tool or die so that the desired inclination is produced inthe final workpiece. The grinding operation only along the lines 5, asshown in FIG. 2a, is then necessary to provide the tablet 40 with anaccurate cylindrical outer dimension. From FIG. 2 it is apparent thatthe flat bottom end surface 7 is provided with a slight depressionsimilar to that of FIG. Ia, which howeveris not detrimental. Subsequentmachining or processing on the end faces of the tablet 40 according toFIG. 2a is unnecessary.

FIG. 3 shows a fragmentary longitudinal section of a nuclear fuel rod.The fuel or breeder material tablets 40 which lie closely one on top ofthe other are surrounded by a tubular can or jacket 8. The size of theangle a is only minimal and is not to be compared with the chamfering ofthe edges, as has been performed heretofore. It is essential that thecontact lines a between the individual tablets 40 of a fuel or breedermaterial column within a fuel rod are located at such a distance fromthe marginal zones proper thereof that the compressive forces which canobviously also occur thereat can be absorbed by a spatial region of thetablet having adequate strength, and can thereby prevent a fracture ofthe tablet. The radial gaps b thereby formed in a column of fuel orbreeder material tablets 40 are very small. The are normally in theorder of magnitude of up to several tenths of a millimeter. Therespective gaps b must be kept very small, for example, if the thicknessof the tubular can wall 8 is very small, so that no part of the tubularcan material can become deformed within these gaps b due to the outerpressure of coolant circulating around the fuel rod. Obviously, it canalso be conversely desirable to provide somewhat larger gaps b so as toproduce the deformation of the tubular can therewith, for example so asto provide a better grip for the tablets 40 within the tubular can 8.Due to the construction of such a conical inclination at the end facesof the tablet 40, the contact lines or the contact points a of thetablets are shifted further radially inwardly so that considerablygreater forces are necessary to be applied before fracture will occur.Such tablets even under axial compressive stresses can be rotated withtheir end faces opposite to one another without detrimentalconsequences. Rejects which are formed normally in the conventionalhandling of tablet columns, for example in the grinding operation, inpackaging or transporting and the further machining or processing offormedfuel rods, are thus considerably reduced thereby, since virtuallyno splitting of the edges occurs any more during insertion of columns ofthe fuel or breeder material tablets into the tubular can or jacketaccording to the invention. Also when the finished fuel rods aresubjected to bending stresses wherein the column of tablets is for themost part prestressed by a spring, a substantially stable behavior oftablets is produced due to the increase of the resistance to fracturethereof at the further radially inwardly located contact edges a.

The new shape of the fissionable fuel or breeder material tablets 40according to my invention, which can for example have a diameter of 5 to20 mm and whose inclined rim has a width of about 0.5 to 2 mm dependingupon the type of material, the strength the density and the like of thetablet, affords very great advantages both in manufacturing techniquesas well as in the-technical uses thereof, which have been confirmed byactual practice to the fullest extent.

The term tablet employed herein is often referred to in the art aspellet and is deemed to be synonymous therewith.

I claim:

1. Ceramic fissionable fuel or breeder material tablet axially alignablewith corresponding tablets for forming nuclear reactor fuel rods,comprising a substantially cylindrical member having a depression atopposite end faces thereof, said end faces respectively including amarginal zone of slight sloping obtusely conical shape, the slope havingan angle of inclination from the horizontal from a minimum of 0.5 up toa maximum of 30 extending and sloping radially outwardly from the outeredge of said depression, said obtusely inclined marginal zone definingwith said outer edge of said depression an annular line-like contactedge for contacting an end face of another tablet axially alignabletherewith, said annular line-like contact edge forming the apex of saidconical shape, being spaced from the peripheral edge of said end face adistance of the order of 10 percent of the diameter of the tablet sothat compression forces acting on said annular line-like contact edge,when the tablet is in use in a nuclear fuel rod, are absorbable by aspatial region of the tablet having adequate strength for absorbing saidforces, and whereby any planar annular portion on said end face areavoided.

2. Tablet according to claim 1, wherein said depression has a flat base.

3. Tablet according to claim 1, wherein said depression has a concavebase.

4. Tablet according to claim 1, wherein said annular contact edge isformed with an obtuse-angled cross section.

5. Tablet according to claim l, wherein said annular contact edge isformed with a substantially circular cross section.

6. Tablet according to claim 1, wherein said annular contact edge isspaced about 0.5 to 2 mm from said peripheral edge of said end face.

* k =F l'-

2. Tablet according to claim 1, wherein said depression has a flat base.3. Tablet according to claim 1, wherein said depression has a concavebase.
 4. Tablet according to claim 1, wherein said annular contact edgeis formed with an obtuse-angled cross section.
 5. Tablet according toclaim 1, wherein said annular contact edge is formed with asubstantially circular cross section.
 6. Tablet according to claim 1,wherein said annular contact edge is spaced about 0.5 to 2 mm from saidperipheral edge of said end face.